The present invention relates to near stoichiometric magnesium oxide dynodes and to a method for preparing the dynodes.
The use of magnesium oxide as a dynode in an electron multiplier is well known. Dynodes are characterized by their ability to emit a plurality of secondary electrons for every incident primary electron. The secondary electron emission coefficient, .delta., which is the ratio of the number of secondary electrons per primary electron, is a measure of the efficiency of the dynode. Obviously a large .delta. is desirable since this reduces the number of stages of dynodes required for a given total electron multiplication. Heretofore, magnesium oxide dynodes have been made by a number of methods. One such method is described in U.S. Pat. No. 2,784,123 issued to P. Rappaport. That patent teaches the making of an MgO film on a AgMg metal alloy base by exposing the AgMg metal alloy to an oxidizing gas, such as water vapor, alcohol, carbon dioxide or nitrogen pentoxide. The AgMg metal alloy with a surface layer of MgO is then heated and exposed to oxygen. Another method is the oxidation of a 1000A thick Mg film at about 400.degree. C. All of the foregoing methods, however, suffer from the drawback that the secondary electron emission coefficient .delta. of an MgO dynode prepared by these methods decreases in value with increase usage (see, e.g. "Preparation and Properties of Thin Film MgO Secondary Emitters" by P. Wargo, V. V. Haxby and W. G. Sheperd, J. Appl. Phys., Vol. 27, p. 1311 (1956)).